1. Field of the Invention
The present invention relates to an insert rotary cutter having at least one indexable cutter insert; and more particularly to a cutter having a cutter insert of the type which includes a plurality of corner cutting edges each formed between a main cutting edge and an auxiliary cutting edge.
2. Prior Art
FIGS. 1A to 1C illustrate one conventional insert rotary cutter for processing a surface of a metal workpiece. The cutter includes a generally disk-shaped body 10 having an axis R of rotation therethrough. The body 10 has a plurality of pockets or recesses 12 disposed adjacent a forward end face 14 thereof in circumferentially spaced relation to one another, each recess 12 opening generally radially outwardly of the body 10. Received in and secured by suitable clamp means (not shown) to each recess 12 is an indexable cutter insert 16 of a hard wear resistant material, as best shown in FIGS. 2A to 2D and FIG. 3. The cutter insert 16 comprises a plate 18 of a generally square shape defined by a front face 20, a rear face 22 disposed parallel to the front face 20, and four side faces 24. Each corner portion of the plate 18 where adjacent side faces 24 intersect each other is removed to provide a first chamfered corner face 26 and a second rounded corner face 28, the first corner face 26 and the second corner face 28 being disposed adjacent each other. The plate 18 has four main cutting edges 30 each defined by the front face 20 and a respective one of the side faces 24, four auxiliary cutting edges 32 each defined by the front face 20 and a respective one of the first corner faces 26, and four corner cutting edges 34 each defined by the front face 20 and a respective one of the second corner faces 28. Each side face 24 is sloping inwardly in a direction away from the front face 20 and serves as a flank of a respective one of the main cutting edges 30 with a positive clearance angle A. Also, each first corner face 26 is sloping inwardly in a direction away from the front face 20 and serves as a flank of a respective one of the auxiliary cutting edges 32 with a positive clearance angle B. Further, each second corner face 28 is sloping inwardly in a direction away from the front face 20 and serves as a flank of a respective one of the corner cutting edges 34 with a positive clearance angle C. The clearance angle B of the flank 26 of each auxiliary cutting edge 32 is selected to be not less than the clearance angle A of the flank 24 of each main cutting edge 30. This is because the cutting loads acting on the auxiliary cutting edges 32 are smaller than those acting on the main cutting edges 30, so that the auxiliary cutting edges 32 require less strength than the main cutting edges 30, and hence it is possible to use larger clearance angles B for the auxiliary cutting edges 32 to obtain good cutting performance. In addition, the corner cutting edges 34 are provided mainly for preventing the main and auxiliary cutting edges 30 and 32 from being damaged, and the clearance angle C of the flank 28 of each corner cutting edge 34 is conventionally selected to be not less than the clearance angle A of the flank 24 of each main cutting edge 30 and not more than the clearance angle B of the flank 26 of each auxiliary cutting edge 32 simply because each corner cutting edge 34 is, as shown in FIG. 2A, disposed between the main cutting edge 30 and the auxiliary cutting edge 32. Thus, the clearance angles A, B and C are so selected as to satisfy A.ltoreq.C.ltoreq.B. Each cutter insert 16 is disposed in such a manner that one set of main, auxiliary and corner cutting edges 30, 32 and 34 are indexed in their machining positions, respectively, and that suitable corner angle D, true rake angle E and inclination angle F are, as shown in FIGS. 1A to 1C, provided to obtain good cutting performance, these angles D, E and F being selected to be positive.
In the prior art rotary cutter aforementioned, a circumferential clearance angle of each indexed corner cutting edge 34 with respect to the workpiece, which will be hereinafter referred to as an effective clearance angle Ce, is, as shown in FIGS. 4 to 6, smaller than a circumferential clearance angle of each indexed main cutting edge 30 with respect to the workpiece, which will be also referred to as an effective clearance angle Ae. As a result, the indexed corner cutting edge 34 is subjected to larger flank wear than the indexed main cutting edge 30 is, so that the corner cutting edge 34 is susceptible to damage earlier than the main cutting edge 30, thereby the service life of the insert 16 becoming short. In addition, the machined surface of the workpiece is liable to be rough.